697 research outputs found
Mathematical modeling of heat and mass transfer in a thermal protection coating with gas flow fluctuations
The thermochemical destruction of a carbon fiber-reinforced polymer when affected by a high enthalpy fluctuating gas flow is simulated numerically. The possibility of controlling the heat transfer process in the composite material is studied
Mathematical simulation of the influence of surface roughness and mass loss on thermal protection
A mathematical model of the thermochemical destruction of carbon fiber-reinforced plastic in the presence of surface roughness and composite ablation is refined based on known theoretical results. It is shown that mass loss through rough surface ambiguously influences the intensity of heat exchange processes in thermal protective material. Results of numerical calculations are compared with known data
Numerical study of the effect of rotation on the behavior of the conjugate heat and mass transfer on the surface of a spherically blunted cone exposed to a hypersonic flow at an angle of attack with ablation from the surface
The processes of heating a body in a high-enthalpy spatial flow with allowance for body rotation around its longitudinal axis and ablation of the thermal protection material are studied by means of mathematical simulation. The obtained solution of the problem in conjugate formulation allowed us to take into account the effect of nonisothermic characteristics of the shell on the heat and mass transfer in the boundary layer
Weak splittings of quotients of Drinfeld and Heisenberg doubles
We investigate the fine structure of the simplectic foliations of Poisson
homogeneous spaces. Two general results are proved for weak splittings of
surjective Poisson submersions from Heisenberg and Drinfeld doubles. The
implications of these results are that the torus orbits of symplectic leaves of
the quotients can be explicitly realized as Poisson-Dirac submanifolds of the
torus orbits of the doubles. The results have a wide range of applications to
many families of real and complex Poisson structures on flag varieties. Their
torus orbits of leaves recover important families of varieties such as the open
Richardson varieties.Comment: 20 pages, AMS Late
Density of States and Conductivity of Granular Metal or Array of Quantum Dots
The conductivity of a granular metal or an array of quantum dots usually has
the temperature dependence associated with variable range hopping within the
soft Coulomb gap of density of states. This is difficult to explain because
neutral dots have a hard charging gap at the Fermi level. We show that
uncontrolled or intentional doping of the insulator around dots by donors leads
to random charging of dots and finite bare density of states at the Fermi
level. Then Coulomb interactions between electrons of distant dots results in
the a soft Coulomb gap. We show that in a sparse array of dots the bare density
of states oscillates as a function of concentration of donors and causes
periodic changes in the temperature dependence of conductivity. In a dense
array of dots the bare density of states is totally smeared if there are
several donors per dot in the insulator.Comment: 13 pages, 15 figures. Some misprints are fixed. Some figures are
dropped. Some small changes are given to improve the organizatio
Ge quantum dot arrays grown by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface: nucleation, morphology and CMOS compatibility
Issues of morphology, nucleation and growth of Ge cluster arrays deposited by
ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered.
Difference in nucleation of quantum dots during Ge deposition at low (<600 deg
C) and high (>600 deg. C) temperatures is studied by high resolution scanning
tunneling microscopy. The atomic models of growth of both species of Ge
huts---pyramids and wedges---are proposed. The growth cycle of Ge QD arrays at
low temperatures is explored. A problem of lowering of the array formation
temperature is discussed with the focus on CMOS compatibility of the entire
process; a special attention is paid upon approaches to reduction of treatment
temperature during the Si(001) surface pre-growth cleaning, which is at once a
key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array
formation process. The temperature of the Si clean surface preparation, the
final high-temperature step of which is, as a rule, carried out directly in the
MBE chamber just before the structure deposition, determines the compatibility
of formation process of Ge-QD-array based devices with the CMOS manufacturing
cycle. Silicon surface hydrogenation at the final stage of its wet chemical
etching during the preliminary cleaning is proposed as a possible way of
efficient reduction of the Si wafer pre-growth annealing temperature.Comment: 30 pages, 11 figure
Island model with genetic algorithm for solution of crystal structure from X-ray powder diffraction data
In this paper, we consider the problem of the study of polycrystalline substances: restoration of a substance atomic structure by full-profile analysis of powder diffraction data. This task is specific since it is not necessary to find very good solutions on average, but it is necessary to find the best one at least sometimes. To solve this problem, it is proposed to use an evolutionary algorithm based on the cooperative island model. The article describes the main stages and features of the algorithm and notes the qualitative advantages of this model in comparison with other methods (including evolutionary). The description of innovations proposed and the results of computational experiments are given. Conclusions from the experimental results are given, and further prospects for improving the efficiency of this method were noted
Floristic Phenomena of the Samara Bend: The Fractal Organization of Taxonomic Diversity
Considering the problem of taxonomic diversity as a fractal object is the aim of this article. The prerequisites for such an approach were articles with varying degrees of detail and argumentation that substantiate taxonomic diversity from the standpoint of fractal geometry. Common to these papers is that the authors in their theoretical constructs start from the Willis rule (law) describing the rank distribution of the relationship between the number of taxa and their volume. The flora of the Samara Bend (the bend of the Volga River in its middle reaches) has become an object of the research. The authors distinguish seven basic floristic areas on the Samara Bend, the boundaries of which coincide with the respective landscapes. The authors discuss the efficiency of the Willis rule (law), which approximates the relationship between the number of taxa and their volume by rank distribution. The multifractal spectrum (a generalized geometric image of generic structure) of the taxonomic diversity of vascular plants of the Samara Bend is presented.
Keywords: taxonomic diversity, fractal organization, Samara Ben
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